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By Design: Designing with ABS

ABS is a good material for applications where lower-cost commodity plastics do not provide the required combination of impact strength and stiffness. In this bimonthly column, Glenn Beall of Glenn Beall Plastics Ltd. (Libertyville, IL) shares his special perspective on issues important to design engineers and the molding industry.

ABS is a good material for applications where lower-cost commodity plastics do not provide the required combination of impact strength and stiffness.

In this bimonthly column, Glenn Beall of Glenn Beall Plastics Ltd. (Libertyville, IL) shares his special perspective on issues important to design engineers and the molding industry.

If you used your telephone today you touched an injection molded ABS part. The excess polystyrene capacity available following World War II resulted in modifications of that material to capture a broader market. A 1948 patent was granted to U.S. Rubber Co.’s L.E. Daley for an ABS type of material. These first ABS plastics were mechanical blends. They found their largest market in extruded pipe and molded fittings. These were not the processor-friendly materials of today.

Continuing research in the 1950s allowed the styrene-acrylonitrile copolymer to be grafted onto the crosslinked polybutadiene. With that breakthrough ABS became acceptable for many applications. Production of ABS was 65 million lb in 1960, 550 million lb in 1970, 1 billion lb in 1980, 1.161 billion lb in 1990, 1.451 billion lb in 2000, and 1.315 billion lb in 2002. ABS is a terpolymer identified as acrylonitrile-butadiene-styrene. This is a mouthful, so the material is universally referred to as ABS. Acrylonitrile provides resistance to heat, chemicals, and aging. Butadiene accounts for ABS’s toughness and low-temperature impact strength. Styrene contributes stiffness, surface luster, ease of processing, and low cost. Changing the type and amount of these three ingredients allows ABS to be adapted for a variety of end-use requirements.

ABS was originally created as a bridge material to fill the gap between low-cost commodity materials such as PE and PS, and higher-cost PC, nylon, and acetal engineering plastics. The list price for large quantities of ABS is $.62 to $1.05/lb for special grades, or $.024 to $.039/cu in.

ABS is an amorphous thermoplastic known for its rigidity, dimensional stability, lustrous abrasion-resistant surface, color consistency, ease of processing, and midrange cost. Its defining characteristic is its notched Izod impact strength of 2.3 to 12 ft-lb/in, coupled with its good balance of other properties. Most ABS compounds are opaque, but there are a few transparent grades with light transmission of 72% to 80%.

Applications

It is now possible to vacuum metallize or electroplate most materials. ABS was, however, the first plastic that formed a secure bond to electroplated coatings. This led to two large markets: appearance-type plumbing applications such as water faucets, and automotive trim parts including grilles and wheel covers. Plumbing and automotive are still major markets, but the interior trim business has been replaced by polypropylene thanks to carmakers’ efforts to reduce cost (and quality?). The large pipe and fitting market has been eroded by lower-cost PVC.

Other major markets include office furniture and machine housings; household furniture; appliance and electronics housings; floor care products; refrigerator liners; luggage; toys; and recreational products such as camper bodies, boat hulls, and snowmobile shrouds. Medical products represent a significant market. Large quantities of ABS are extruded into sheet for thermoforming.

Specifics of ABS design
  • Wall thicknesses of .010 inch have been molded, but ABS is not an ideal material for thin-walling. A better minimum thickness is in the range of .030 to .040 inch. Part thicknesses greater than 1.000 inch have been successfully molded using a warm mold with large gates and runners. The ideal compromise between cost and quality will be realized with a maximum wall thickness of .250 inch.
  • Radiusing. ABS is not a notch-sensitive material; however, radiusing the corner of an injection molded part improves its strength by distributing corner stresses over a broader area. Inside corner radiuses should be limited to not less than 25% of the part’s wall thickness. For maximum strength the radius should be 60% of wall thickness. Larger radiuses can be specified, but this will not significantly increase part strength.
  • Molding draft angles. ABS is a rigid, but relatively soft, material that can often be molded with no draft angle. However, a .5° to 1° draft angle per side will shorten cycle time and make a better part.
  • Projections of all types can be molded with ABS. The thickness of projections can be 75% of wall thickness. In instances where sink marks cannot be tolerated, the thickness of projections should be reduced to 50% to 60% of wall thickness.
  • Depressions and holes. Strong weldlines that do not distract from a part’s appearance can be produced with proper molding conditions. Inside corners on irregularly shaped holes must be radiused to avoid molded-in stress. Draft angles on holes will improve quality and shorten cycle time. The depth of holes should be limited to two and one-half to three times the thickness of the core to avoid core pin deflection.
  • Tolerances. ABS is a low-shrinkage, dimensionally stable, amorphous material with uniform shrinkage. A “commercial” tolerance for a .125-inch-thick, 1.000-inch-long ABS part would be ±.0045 inch. A more costly “fine” tolerance could be ±.0023 inch. Parts that are less than .125 inch thick can be held to a slightly closer tolerance. Thicker parts, which take longer to cool and shrink more, may require a larger tolerance.
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